In production of a semiconductor die, imaging of the wafer upon which the dice are formed is an integral part of the fabrication process, and images are typically generated for many stages of the fabrication process. Typically, the wafer images are used for the purpose of inspection and/or quality control of the specific stages. One of the methods used to produce the images is to scan the wafer, and form the scanned images, and/or determine characteristics of the section of the wafer being scanned, from radiation returning from the section. The scanning process is relatively time-consuming, and methods for reducing the scanning time, while maintaining the quality of the received signals, are well known in the semiconductor art. One such method is to use apparatus that performs multiple scans simultaneously. A number of other methods and techniques have also been used for enhancing the production of the image of a scanned object. The references below describe some of these methods and techniques.
U.S. Pat. No. 5,355,252 to Haraguchi, whose disclosure is incorporated herein by reference, describes a scanning laser microscope using a single beam.
U.S. Pat. No. 6,674,522 to Krantz et al., whose disclosure is incorporated herein by reference, describes an optical technique for inspecting photomasks. The techniques are based on multiple modified radiation collection techniques.
U.S. Pat. No. 6,853,475, to Feldman et al., whose disclosure is incorporated herein by reference, describes a method for producing multiple optical beams which are scanned across the surface of a wafer. The method uses an acousto-optical device wherein multiple traveling lenses are generated, each lens forming a respective traveling beam which is focused onto, and which is scanned over, the wafer surface.
An article entitled “Bright field-bright future: Material defect detection with a laser scanning system,” by Larson et al., published in Sep. 1997 in Solid State Technology, is incorporated herein by reference. The article describes splitting a single laser beam into two correlated beams. The two beams irradiate a surface, and the returning radiation is combined into one beam. The recombined beam provides information on the surface. The system uses a differential interference contrast (DIC) technique, first described by Nomarski et al. in Rev. Metallurgie L11, 121, 1955. In DIC the prism used to split the beam is a Wollaston prism, or a Nomarski prism.
Basic confocal microscopy principles were described in U.S. Pat. No. 3,013,467, to Minsky, whose disclosure is incorporated herein by reference. In a confocal microscope, a light beam is focused to a spot in the object plane, and this spot is imaged onto a small circular aperture (often a pinhole) placed in front of a detector. Confocal microscopy improves the discrimination of objects in the focal plane compared with those not in the plane.
In articles titled “Scanning mirror microscope with optical sectioning characteristics: applications in ophthalmology” by C. J. Koester, published in Appl. Opt., 19, pgs 1749-1757, 1980, and “Confocal Microscopes with slit apertures” by C. J. R. Sheppard et al., published in J. Mod. Opt. 35, pgs 1169-1185 (1988), the use of a slit (“one-dimensional confocal”) instead of a small circular aperture is described. Both articles are incorporated herein by reference. The use of a slit source with a slit aperture allows a larger signal to be detected, compared with a circular aperture with a diameter of the size of the slit width.
U.S. patent application Ser. No. 2005/0225849 to Gouch, whose disclosure is incorporated herein by reference, describes a confocal microscope having a slit source. The slit source is focused onto an object, and radiation from the object is focused onto a linear array of detectors.
U.S. Pat. No. 5,241,364 to Kimura, whose disclosure is incorporated herein by reference, describes a confocal phase contrast scanning microscope. The optics of the microscope includes an annular phase plate, via which a collimated beam is passed, before being focused to a point on an object. Light from the object is focused onto the entrance of a fiber optic, which transfers the received light to a detector. Scanning is performed by mechanically moving the optics and the object independently.
In an article titled “Tandem-scanning reflected-light microscope” by Petran et al., published in J. Opt. Soc. Am., 58, pgs 661-664 (1968), whose disclosure is incorporated herein by reference, methods are proposed to allow simultaneous detection of signals from a large number of apertures in a scanning confocal microscope. The methods include using aperture arrays and Nipkow discs.
Other configurations of scanning confocal microscopes for inspection applications have been proposed. Examples are provided in U.S. Pat. No. 6,248,988 to Krantz, U.S. Pat. No. 6,429,897 to Derndinger et al., and U.S. patent application Ser. No. 2003/0156280 to Reinhorn, all of whose disclosures are incorporated herein by reference.